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JavaScript API

crypto

SubtleCrypto

deriveBits

Open source

deriveBits

It takes as its arguments the base key, the derivation algorithm to use, and the length of the bits to derive. It returns a Promise which will be fulfilled with an ArrayBuffer containing the derived bits. This array of bits can be used as a key for encryption or decryption as a shared secret.

Usage

deriveBits(algorithm, baseKey, length)

Parameters

Name	Type	Description
`algorithm`	EcdhKeyDeriveParams or Pbkdf2Params	An object defining a derivation algorithm to use.
`baseKey`	CryptoKey	Represents an input to derivation algorithm. Could be an ECDH private key or a PBKDF2 password imported as a CryptoKey.
`length`	`number`	The length of the bits to derive. Must be a positive number and a multiple of 8.

Supported algorithms

ECDH	PBKDF2	HKDF
✅ EcdhKeyDeriveParams	✅ Pbkdf2Params	❌

Return Value

A Promise that resolves to a new ArrayBuffer containing the derived bits.

Throws

Type	Description
`OperationError`	Raised when `length` is 0 or not a multiple of 8.

Examples

Deriving bits using ECDH

export default async function () {
  // Generate a key pair for Alice
  const aliceKeyPair = await crypto.subtle.generateKey(
    {
      name: 'ECDH',
      namedCurve: 'P-256',
    },
    true,
    ['deriveKey', 'deriveBits']
  );

  // Generate a key pair for Bob
  const bobKeyPair = await crypto.subtle.generateKey(
    {
      name: 'ECDH',
      namedCurve: 'P-256',
    },
    true,
    ['deriveKey', 'deriveBits']
  );

  // Derive shared secret for Alice
  const aliceSharedSecret = await deriveSharedSecret(aliceKeyPair.privateKey, bobKeyPair.publicKey);

  // Derive shared secret for Bob
  const bobSharedSecret = await deriveSharedSecret(bobKeyPair.privateKey, aliceKeyPair.publicKey);

  console.log('alice shared secret: ' + printArrayBuffer(aliceSharedSecret));
  console.log('bob shared secret: ' + printArrayBuffer(bobSharedSecret));
}

async function deriveSharedSecret(privateKey, publicKey) {
  return crypto.subtle.deriveBits(
    {
      name: 'ECDH',
      public: publicKey,
    },
    privateKey,
    256
  );
}

const printArrayBuffer = (buffer) => {
  const view = new Uint8Array(buffer);
  return Array.from(view);
};

Deriving bits using PBKDF2

export default async function () {
  const password = stringToArrayBuffer('my secret password');

  // Import the password as a key
  const baseKey = await crypto.subtle.importKey('raw', password, 'PBKDF2', false, [
    'deriveBits',
    'deriveKey',
  ]);

  // Generate a random salt
  const salt = crypto.getRandomValues(new Uint8Array(16));

  // Derive 256 bits using PBKDF2
  const derivedBits = await crypto.subtle.deriveBits(
    {
      name: 'PBKDF2',
      hash: 'SHA-256',
      salt: salt,
      iterations: 310000,
    },
    baseKey,
    256
  );

  console.log('derived bits: ' + printArrayBuffer(derivedBits));
}

function stringToArrayBuffer(str) {
  const buf = new ArrayBuffer(str.length * 2);
  const bufView = new Uint16Array(buf);
  for (let i = 0, strLen = str.length; i < strLen; i++) {
    bufView[i] = str.charCodeAt(i);
  }
  return buf;
}

const printArrayBuffer = (buffer) => {
  const view = new Uint8Array(buffer);
  return Array.from(view);
};